Testing method of thermal spray powder particle size and particle size distribution

       The main detection methods of thermal spray powder particle size and particle size distribution are:



(1) Sieve analysis method The sieve analysis method is to use a vibrating sieve machine to pass the powder through a standard sieve woven by metal wires, weigh the weight of the sieved powder respectively, and calculate the percentage of the sieved weight. The specifications of the standard sieve can be selected according to the powder use, that is, according to the spraying (welding) process and equipment characteristics.



(2) Sedimentation balance method Sedimentation balance method is suitable for the determination of powder particle size composition in the range of 1~100um. Method gist: The powder is prepared into a liquid suspension, placed in a measuring instrument, and the powder particles slowly settle in the still liquid under the action of gravity. Due to the different particle sizes and the different sedimentation rates, the sedimentation curve (that is, the curve of the accumulated weight of sedimented particles versus time) is drawn with the help of the automatic recording device of the sedimentation balance, and the particle size composition of the powder is analyzed from the sedimentation curve.



(3) Light transmission method Light transmission method is also known as turbidimetric method, and the particle size range can be as small as 0.1um. Method points: Prepare the particle suspension, put it in a container, put the container in front of the light path window, and measure the attenuation of light intensity when light passes through the suspension. The percentage of particle size is the ratio of the logarithmic difference in light intensity corresponding to two adjacent times multiplied by the average of the Stokes diameters at the two times and its sum.



(4) Elutriation method Elutriation is the reverse process of sedimentation, that is, the process of particle classification by means of fluid flowing in the opposite direction. Method points: The powder sample is placed in the U-shaped glass tube of the Roller instrument and sent to the fluid of known velocity. The particles smaller than the separation size enter the sleeve through the sedimentation chamber and weighed; the larger particles remain in the U-shaped tube. And rotate, only when the fluid velocity is greater than the particle settling velocity, the particles in the liquid can rise without falling, and the particle size is calculated according to the final fluid velocity suitable for separating a certain particle size. This method is no longer commonly used.



(5) Laser method Laser method is a new method for particle size measurement developed in the past 20 years. The common ones are laser diffraction method and photon coherence method.



In the late 1970s, the laser particle size analyzer developed according to the Fraunhofer diffraction theory appeared, which has the advantages of good repeatability and fast measurement speed. This instrument has a lower measurement limit of a few microns and an upper limit of 1000um. The disadvantage is that the error of the instrument is still large for samples of several microns. In the mid-1980s, new theories emerged that corrected this, improving the accuracy of measurements in the small particle size range.



The measurement range of laser particle size analyzer is generally 0.5 ~ 1000um. Concentric multi-element photodetectors were used to measure the scattered light intensity at different line angles, and then the particle size distribution was calculated according to the above theory.



One advantage of this method is that it is suitable for on-line measurement, especially for the measurement of droplet size distribution. The disadvantage is that the calculation is very cumbersome, but with the development of computers, this disadvantage can be overcome. In general, the laser method is not as accurate as the sedimentation method.